A DC motor is an electric motor that can run on direct current (also known as DC-current). Low power (e.g. <1 kW) DC motors are used in many applications (e.g. toys, disk drives, etc), and can operate directly from batteries. When a constant voltage is applied to a DC-motor, it will typically rotate at a constant speed (optionally after a transient behaviour). During this rotation a voltage called “back-emf” is induced in the rotor windings, which compensates the applied voltage.
DC-motors are not always used for continuous rotation, but also for position control, e.g. for opening or closing a valve, or setting the valve opening at a specific value. Circuitry has been developed for driving the DC motor to the desired position, and for keeping the DC-motor in that position, even when an external force is applied. Such circuits are typically called “servo systems”. In order to set and maintain a desired position, the servo system needs a signal indicative of the actual motor position. Servo control can then e.g. be performed by using this position signal in a negative feedback-loop, where a difference signal is calculated between the desired position and the actual position, which difference signal may optionally be amplified, and applied to the DC-motor. Such circuits are well known in the art.
U.S. Pat. No. 5,705,907 describes a drive control system for limiting overshoot when the actual position of the DC-motor reaches a target position. A block diagram of such a circuit is replicated in FIG. 1. It has a CPU 104 sending a PWM signal as a command-signal to a driver 105, which delivers power to a motor 101. A disadvantage of such motor control system is that the motor module 102 actually requires five wires, which implies additional handling and material cost. This may be important in applications where the distance between the CPU 104 and/or driver 105 on the one hand, and the motor 101 on the other hand is relatively large (e.g. larger than 50 cm), especially in high-volume, cost sensitive applications, such as e.g. air-conditioning systems for vehicles.
EP0415655 describes a two-wire power and position control system between a central station 100 (i.e. the motor controller) and a remote station 102 (i.e. the motor module). FIGS. 2A, 2B and 2C of this document correspond to FIGS. 5 (left part), FIG. 7 and FIG. 8 of EP0415655(B1), respectively. A disadvantage of this circuit is that the motor can only be driven in a single direction, which makes this circuit unsuited for many practical applications.